Maintaining proper air pressure in a pneumatic tire is very important to enable the tire to properly perform desired functions.
To maintain air pressure inside of a pneumatic tire, its inner surface is typically comprised of a relatively thin rubber layer intended to prevent or retard the permeation of air into the tire carcass from the tire's inner air chamber. It is often referred to as an innerliner. Innerliners have been used for many years in tubeless pneumatic vehicle tires to retard or prevent the escape of air used to inflate the tire, thereby maintaining tire pressure. Rubbers which are relatively impermeable to air are often used as a major portion of said innerliners and can include butyl rubber and halobutyl rubbers such as, for example, chlorobutyl rubber and bromobutyl rubber.
However, such elastomers are typically the most expensive rubbers used in the tire.
While alternative, less expensive, elastomers may be used for the innerliner, such as for example styrene/butadiene rubber (SBR), natural rubber (NR) and cis 1,4-butadiene rubber (BR), a significantly thicker gauge of the alternate elastomers would be required for a similar air retention effect, leading to an increased tire weight.
To reduce both the weight and cost of the tire innerliner, it is proposed to utilize an inclusion of hollow glass microspheres in the tire innerliner rubber composition which have a significantly lower specific gravity than butyl rubber and halobutyl rubbers.
However, it is considered herein that a simple inclusion of a dispersion of hollow glass microspheres in the tire innerliner would not be sufficient to meet desirable performance standards when the tire, hence the tire inner liner, is cyclically deformed during tire running conditions. Such flexural deformation of the tire innerliner is likely to promote separation of the hollow glass microspheres from the elastomer matrix, hence, creating a passage for air to go through the inner liner rubber layer and to thereby reduce its air impermeability.
Without adequate bonding of the microspheres to the halobutyl rubber matrix and/or reinforcing filler system, they would form voids following high strain application wherein the voids would both create a path for air permeation but also a fracture area to create cut growth and potential tire innerliner cracking.
Accordingly, it is desired to prepare a more stable matrix for the inner liner rubber composition.
For this invention it is envisioned that a butyl rubber (e.g. halobutyl rubber) matrix, particularly a tire innerliner butyl rubber matrix, be provided which contains a dispersion of the hollow glass microspheres which are anchored within the innerliner butyl rubber based composition to the butyl rubber itself to inhibit, or restrict, their mobility within the butyl rubber composition, when cured, to promote a more effective air permeation resistant barrier layer for the tire during service of the tire under operating conditions. Such innovation is considered herein to be in a form of a new butyl rubber matrix, or in a sense a network, in a form of a tire inner liner layer which is a significant departure from, and readily distinct from, a tire inner liner containing a simple inclusion of a dispersion of hollow glass microspheres.
In practice, a tire rubber inner liner may, for example, be prepared by conventional calendering or milling techniques to form a strip of uncured compounded rubber of an appropriate width. Typically the innerliner rubber strip is the first element of the tire to be applied to a tire building drum, over and around which the remainder of the tire is built. When the tire is cured, such inner liner becomes an integral, co-cured, part of the tire. Tire innerliners and methods of preparation are well known to those having skill in such art.
In the description of this invention, the terms “rubber” and “elastomer”, where used, are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize”, where used, are used interchangeably unless otherwise indicated. The terms “compound”and “rubber composition”, where used, are used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of a particular material per 100 parts by weight rubber.